Series Load Resonant (SLR) Converter

Contents

Description

The Series Load Resonant converter is a DC/DC converter based on a resonant circuit which allows soft-switching operation. In a soft-switching converter, switching occurs when voltage and/or current values are zero, thus significantly improving converter's efficiency. A switch is said to have zero-voltage switching (ZVS) and zero-current switching (ZCS) when the voltage/current is zero as the switch changes state.

The converter consists of a full-bridge MOSFET bridge, a resonant tank represented by the Lr inductor and Cr capacitor block, a full-bridge rectifier and an output filter (Co). The resonant frequency of the tank is given by:

The switching frequency range determines the three possible operating modes of the SLR converter:

Discontinuous Conduction Mode (DCM), where , leading to a ZVS and ZCS at turn-off and a ZCS at turn-on.

Continuous Conduction Mode (CCM1) where , leading to a ZVS and ZCS at turn-off.

Continuous Conduction Mode (CCM2) where , leading to a ZVS and ZCS at turn-on.

Simulation

In this example the resonant frequency = 11.25 kHz. Select a switching frequency = 13.14 kHz (conduction mode 3 = CCM2) and run the simulation. Observe waveforms on Scope 1 and Scope 2. For a 0.5 Ohm load, you should get a 10V output for a 12V input voltage. Now try the other conduction modes and/or change the load from 0.5 Ohm to 5 Ohms. You will see that the output will no longer be 10V due to the DC gain characteristic of the resonant circuit for a given load.

Observe the DC gain characteristic plot (0.5 Ohm traces), you will note that the experimental gain values are slightly shifted relative to the theoretical gain values. This is due to the fact that the output filter Co is not taken into account in the theoretical gain calculation. In addition, as the DC gain characteristic plot suggests (5 Ohms trace), it will be difficult to regulate the output of the SLR converter at light load.

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